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Played a gig Fri. night and no problem with the amp. Had another gig Sat. night and plugged in the amp, using a Variac set to 117v. Verified the Variac with mulitmeter at 117v.
Immediately on warm up, the amp had a loud hum. No controls would stop the hum. Checked all tubes (changed) and same results. I haven't checked out the amp since I got home. Nothing I did had an effect on the loud hum.
Anybody have anything right off the bat. This is my main gigging amp and I've never had a problem with it.

Well heck, I just noticed another thread about a Princeton with hum. I'll check the filter cap but this amp has been serviced within the last 4 years with total cap job. It has been flawless until now.

No, 2 different places. Never had any electrical problems at either place. I always check the power with my voltmeter to make sure of what we have. I also use a Variac and set my power at 117-120v. We have 124v at my house.

Not all that unusual to have a cap failure after a recent cap job due to either a component failure or even more commonly a solder flaw on a recently installed cap. If the volume doesn't change the intensity of the hum, it is most likely due to a cap failure of some kind.

FWIW a Variac is not recommended for maintaining stable voltage over a period of time - it's test tool and is not meant for "set and forget" uses like a power strip. They can drift, be bumped and don't do a thing for you if there's a power spike.

For gig purposes a real voltage regulator with spike protection would be recommended - but only if you play gigs where they supply temporary power from generators or with obviously haphazard wiring.

A $7 outlet tester so you know whether the ground is actually connected and the polarity correct is FAR more useful than a Variac (I check the power source every place I play)

Your amp doesn't need a "not to exceed" level of 117 or whatever - it serves no purpose, but a Variac is potentially more harmful than helpful.

Modern wall voltage that hits 121 or so is perfectly safe for any vintage tube amp - BF, SF, tweeds - doesn't matter. The only differences are in the test voltages on the schematic - they need to be factored for actual supply voltage.

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